Change that builds from one level to the next is described as while development that occurs in distinct stages or steps is

continuous; discontinuous

Explanation:

In continuous change, there is the steady transformation of processes in which what was achieved at a point in the process was built on the milestones from previous processes. A child’s physical growth is an example of a continuous change as the height are being added in incremental inches year after year.  

Development can be discontinuous when it happens in unique stages, which could be at some specified time or age.  The change here happens in a sudden.

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."

an informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.